1. Field of the Invention
The invention relates to a sheet feeding apparatus which is provided to a printer, a facsimile apparatus, a copying apparatus, a typewriter, and other various machines each having a sheet feeding mechanism and which uses a travelling wave as a feeding force (conveying force).
2. Related Background Art
As a conventional mechanism to feed a sheet or the like using a vibration wave, there is a mechanism as shown in FIG. 2.
In FIG. 2, reference numeral 1 denotes a voltage controlled oscillator (VCO) which oscillates at a frequency that is four times as high as a driving frequency of an ultrasonic vibration member, which will be explained hereinafter. Reference numeral 2 denotes a ring counter for counting an output of the VCO 1 and for shifting the timings of the conductive states of switching transistors 4a to 4d at a frequency of 1/4 of the output of the VCO 1 in a manner such that the phase of the switching transistor 4b is deviated by 180.degree. from the phase of the switching transistor 4a, the phase of the switching transistor 4c is deviated by 90.degree. from that of the transistor 4a, and the phase of the switching transistor 4d is deviated by 270.degree. from that of the transistor 4a, respectively. Reference numerals 5a to 5c denote step-up transformers each for raising up to a voltage at which an ultrasonic vibration member can be sufficiently driven. The switching transistors 4a and 4b are connected to the primary side of the transformer 5a. There is a phase difference of 180.degree. between the ON timings of the switching transistors 4a and 4b. Thus, a stepped-up AC wave of a frequency which is 1/4 of the output frequency of the oscillator 1 is generated from the secondary side of the transformer 5a.
The switching transistors 4c and 4d are connected to the primary side of the transformer 5b. There is a phase difference of 180.degree. between the ON timings of the switching transistors 4c and 4d in a manner similar to the case of the transformer 5a. On the other hand, the switching transistor 4c has a phase difference of 90.degree. with respect to the switching transistor 4a. The switching transistor 4d also has a phase difference of 90.degree. with respect to the switching transistor 4b. Consequently, an AC wave whose phase is deviated by 90.degree. from the output of the transformer 5a is generated from the secondary side of the transformer 5b. On the other hand, the transistors 4a and 4b are connected to the primary side of the transformer 5c at the positions opposite to the connecting positions of the primary side of the transformer 5a. Therefore, an AC wave whose phase is deviated by 180.degree. from the output of the transformer 5a is generated from the secondary side of the transformer 5c.
Reference numerals 11 and 12 denote elastic members by which a sheet 8 is sandwiched by a proper pressure. Piezoelectric transducers 6 (6a, 6b) and 7 (7a, 7b) are adhered to the surfaces of the elastic members 11 and 12 on the side opposite to the surfaces of the elastic members 11 and 12 which are in contact with the sheet 8. FIGS. 3A and 3B show a vibrator comprising the piezoelectric transducer 6 and the elastic member 11. Since another vibrator comprising the piezoelectric transducer 7 and the elastic member 12 also has substantially the same construction as the above vibrator, its description and drawing are omitted. FIG. 3A is a perspective view of the vibrator. FIG. 3B shows a polarization pattern of the piezoelectric transducer. In FIG. 3B, a segment containing a (+) polarizing direction which is opposite to that in a segment containing a (-). As described in JP-A-3-93482, the polarization pattern shown in the above-mentioned FIG. 3B is formed so that a gap of the polarization pattern exists in the node of the vibrating mode of each of the A phase and B phase. In the polarization pattern, electrodes 6a.sub.1 to 6a.sub.5 are referred to as A phase and electrodes 6b.sub.1 to 6b.sub.5 are referred to as B phase hereinafter. There is a positional deviation of 1/4 wavelength between the A phase and the B phase. A travelling wave can be formed in the vibrator by delaying or progressing the AC wave which is applied to the B phase by 90.degree. with respect to a time for the AC wave that is applied to the A phase. The progressing direction of the travelling wave is determined in dependence on whether the time phase difference of the B phase with respect to the A phase is delayed or advanced.
The driving principle of the sheet will now be described. FIG. 5 is a diagram showing the principle of the sheet feed. The convex portions of the travelling waves generated on the elastic members 11 and 12 are mutually overlapped. Such an overlapping state be realized by the following method. Namely, the directions of the travelling waves generated on the two upper and lower vibration members when the two elastic vibration members 11 and 12 having the same construction are arranged so as to face each other are set to the opposite directions. Further, a phase difference between the A phases of two vibration members is set to 180.degree. and a phase difference between the B phases of two vibration members is set to 0.degree.. In this instance, within each of the upper and lower vibration members, a phase difference between the A phase and the B phase is set so as to have a delay or advance of 90.degree.. That is, when the A phase of the upper vibration member is used as a reference, there are the following two combinations (1) and (2).
(1) (upper A phase =reference, PA1 (2) (upper A phase=reference,
upper B phase=+90.degree., PA2 lower A phase=180.degree., PA2 lower B phase=+90.degree.) PA2 upper B phase=-90.degree., PA2 lower A phase=180.degree., PA2 lower B phase=-90.degree.)
The feeding directions of the sheet in case of the above combinations (1) and (2) are opposite. FIG. 2 shows a circuit which realizes the combination (2). On the other hand, in case of feeding the sheet 8 in both of the forward and reverse directions, the combinations (1) and (2) are switched by using a switch or the like.
When an attention is paid to a certain mass point (11a in FIG. 5) on the travelling wave generated as mentioned above, the mass point performs an elliptic motion by the travelling wave. For instance, when the travelling wave progresses to the right as shown by an arrow 13, the mass point 11a draws a clockwise elliptic orbit as shown in the diagram. Therefore, the moving direction of the mass point 11a on each of the elastic members 11 and 12 is opposite to the progressing direction of the travelling wave and functions as a feeding force to feed the sheet 8. On the other hand, in the concave portion, a sheet feeding force in the same direction as the progressing direction of the travelling wave is generated. However, since a pressure is smaller as compared with that in case of the convex portion, frictional forces between the sheet 8 and the elastic members 11 and 12 are small and the sheet feeding forces are also small. Consequently, the total force of the sheet feeding forces acts in the direction opposite to the progressing direction of the travelling wave mentioned above.
Reference numeral 9 denotes an encoder which sandwiches the sheet 8 together with a roller 10 and supplies the motion of the sheet 8 to a controller 3. The controller 3 determines a command value, practically speaking, a frequency to the oscillator 1 in accordance with the motion of the sheet 8, thereby controlling the position or speed.
The sheet feeding apparatus using the vibration wave as shown in FIG. 2 has advantages such that a transfer loss and gear noise or the like don't exist and highly accurate sheet feed can be realized as compared with a sheet feeding apparatus of the type in which a driving force from, for example, a motor or the like is transferred to a paper feed roller through a transfer mechanism such as a gear or the like.
However, two vibrators are used to feed the sheet. The AC waves having three kinds of phases are necessary to feed the sheet. Such a number of phases is larger than the number of kinds (two kinds) of phases which are generally necessary to drive one ultrasonic motor. Therefore, the number of transformers or the like is large. In addition, the price and size of the circuit are high.